In supplying RF power to a load side, from an RF supply side such as an RF power source, an impedance matching device is provided for matching RF supply side impedance to load side impedance, thereby enhancing efficiency in supplying the RF power.
Along with producing a finer product by semiconductor producing equipment, a higher degree of stability against plasma load fluctuation is required, for example, prompt response and convergence within a short time, from exciting plasma until stabilization. In order to stabilize power feeding to plasma and to reduce plasma fluctuation, an impedance matching device is needed for enabling high-speed impedance matching.
Generally used impedance matching devices perform mechanical matching operations, by a motor-driven vacuum variable capacitor, and therefore, in some cases, several seconds may be required until completion of the impedance matching. Instead of such mechanical impedance matching devices, electronic impedance device not containing mechanical elements are also suggested. This kind of electronic impedance matching device is referred to as “electronic matcher” (see the patent document 1).
The electronic impedance matching device may be configured by a variable reactor. The variable reactor has a control winding together with a main winding, which are wound around a ferrite core, allowing the current in the control winding to be variable electronically according to a step-down chopper circuit so as to change inductance, and thereby rendering impedance to be variable. According to the variable reactor, the impedance can vary by controlling the current in the control winding, without using any mechanically movable part, providing advantages of speeding-up and maintenance-free.
FIGS. 14A through 14C each illustrates configuration example of the variable reactor that is used in a conventional electronic impedance matching device, FIG. 14A shows a circuit example of the variable reactor, and FIG. 14B shows an example of an EI core structure of the variable reactor.
The variable reactor 102 comprises two types of windings, a control winding 102a and main windings 102b (102b1 and 102b2), being wound around a ferrite core 102c. The control winding 102a is wound around a central part of the ferrite core 102c, and DC current passes therethrough. The main windings 102b are wound respectively around both sides of the ferrite core 102c, and they are fed with RF current, 13.56 MHz, for example, from a high-frequency power source (RF power source) that is connected to the impedance matching device.
The variable reactor 102 has wiring of the main windings being wound around both sides of the control winding as described above, and magnetic fields generated by the main winding 102b1 and the main winding 102b2 are canceled each other at the central part of the ferrite core 102c, thereby achieving a configuration that RF voltage generated by the main windings 102b1 and 102b2 is not induced into the control winding 102a side.
Here, inductance L of the variable reactor is determined according to the following formulas:L=(μ·S/l)·N2  1μ=B/H  2
In the formulas above, μ is magnetic permeability, S is core sectional area, Nis the number of turns of the main winding, l is a magnetic path length, B is magnetic flux density, and H is a magnetic field. The formulas 1 and 2 represent that the inductance L is proportional to the magnetic permeability μ, and the magnetic permeability μ is inversely proportional to the magnetic field H.
The ferrite core used in the variable reactor has nonlinear hysteresis property, and the magnetic permeability μ is represented by a gradient on the B-H curve according to the formula 2.
The formulas 1 and 2 express that when the magnetic field H becomes smaller, the magnetic permeability μ becomes larger, and the inductance L also becomes larger. In addition, a magnitude of the magnetic field H generated in the ferrite core is proportional to DC current passing through the control winding. Therefore, in the electronic impedance matching device, the magnitude of the magnetic field H being generated is made to vary by controlling the DC current Idc passing through the control winding, and variation of the magnitude of the magnetic field H allows the inductance L of the variable reactor to be variable.
On the B-H curve, an AC magnetic field is generated by the RF current passing through the main windings, in addition to the DC magnetic field generated by the DC current passing through the control winding. However, in the variable reactor, when AC magnetic flux density is compared with DC magnetic flux density, in the range of the DC magnetic field generated by the DC current, the AC magnetic flux density is equal to or less than 10% of the DC magnetic flux density, and the magnetic flux density of the variable reactor can be considered as almost depending on the DC magnetic flux density. Therefore, it can be assumed that the magnetic permeability μ is determined by an operating point of the DC magnetic field and DC magnetic flux on the B-H curve. Thus, the inductance can be made variable, by controlling the DC control current to vary the magnetic field so that this variation of the magnetic field allows the magnetic permeability to be variable.
FIG. 14C illustrates a schematic configuration of the impedance matching device using the variable reactor. Here, the variable reactor 102A connected in parallel between input-output terminals varies the absolute value of the impedance, and the variable reactor 102B connected in series between the input-output terminals varies a phase component of the impedance. The impedance control circuit 101 varies the inductance of each variable reactor, thereby matching the impedance on the input terminal side to the impedance on the output terminal side.